Protective shield for providing an impedance match between a radar feed and its parabolic reflector



13, 1957 J. o. SEAMANS 2,803,009

' PROTECTIVE SHIELD FOR PROVIDING AN IMPEDANCE MATCH BETWEEN A RADAR FEED AND us PARABOLIC REFLECTOR Filed NQV. 13. 1950 INVENTOR Jam as. 0. 5 4272023 ATTORNEY United States Patent PROTECTIVE SHIELD F'QR PRQVIDING AN IM- PEDANCE MATCH BETWEEN A RADAR FEED AND ITS PARABOLIC REFLECTOR James 0. Seamans, Winston-Salem, N. (1., assignor to Western- Electric Company, incorporated, New York, N. Y'., a corporation of New York ApplicationNovember 13, 1950, Serial No. 195,243

3 (llaims. (Cl. 343 782) This invention relates to radar scanner shields and more particularly to a shield used in ultra-high frequency antennas systems which employ a rotating or nutating feed and a stationary reflector.

A large number of radar antenna systems have been devised which use stationary parabolic reflectors to transmit energy which is delivered from a rotating or nutating scanner. In all of these systems an inherent impedance mismatch has resulted due to the return of energy from the reflector to the scanner or vice versa. This impedance mismatch degrades the performance of the entire radar system by pulling the magnetron. Attempts to provide an impedance match by the use of a shield be,- tween the scanner and the reflector have met with failure since the minor lobes of the antenna radiation pattern have at the same time been enlarged to distort the radiation field. It is desirable in all radar systems to increase the antenna gain as much as possible and to afford the necessary impedance match without distorting the radiation pattern of the reflector.

It is essential in antenna systems to prevent damage to the nutating mechanism caused by small particles of wind blown debris and the like which would render the nutating mechanism inoperative and would necessitate de-energization of the radar for repairs.

Accordingly, one object of this invention is to provide a shield for a nutating or rotating scanner which will prevent damage to the scanner caused by wind blown debris.

Another object of this invention is to provide a shield which will provide an impedance match between the scanner and the reflector of an ultra-high frequency antenna system.

Another object of this invention is to provide a shield which will match the impedance of the reflector and the scanner and will neither add to nor subtract from the minor lobes inherent in the reflector.

A still further object of this invention is to increase the antenna gain without at the same time distorting the radiation pattern of the reflector.

With these and other objects in View, the invention comprises a continuously curving shield several wave lengths long which is positioned between a nutating or rotating feed and a stationary reflector in an ultra-high frequency antenna system. Energy incident to the shield will be diffused in a plurality of directions to prevent an impedance mismatch between the feed and the reflector and at the same time will prevent distortion of the radiation pattern inherent with the reflector.

Other objects and advantages will be apparent from the following detailed description when considered in conjunction with the accompanying drawings, wherein Figure 1 discloses one embodiment of the invention showing a reflector and a shield in section, and a simplied view of a nutating mechanism;

Fig. 2 is an enlarged front view looking into the mouth of the shield; and

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Fig. 3 is a cross-sectional view of the shield taken along line 3-3 of Fig.2.

Referring now to the drawings, wherein like reference numerals designate the same elements throughout the several views, there is shown in Fig. 1 a stationary parabolic reflector indicated by the number 10. A translation device 11, which may be either a radar transceiver or a microwave receiver or transmitter, is connected by a wave guide 13 to a nutating system, indicated generally as 12. The nutating mechanism 12 includes a motor 14 which drives a shaft 15. Connected to the shaft 15 is a bell crank which is pin coupled to a rotating crosshead shaft 18. The bell crank 20 is linked to a push rod 28 which reciprocates as the hell crank is rotated with the shaft 15. The resultant oscillation of the bell crank 20 deflects an antenna support 17, which is connected to the crosshead shaft 18, back and forth with respect to'the axis of rotation. An antenna feed or feed horn 27, which is connected by means of a gimbal ring 19 to the'antenna support 17, is thus caused to nutate with a spiral scan. A spring 21 on the shaft 15 balances the centrifugal force set up in the rotating and reciprocating parts. At the junction of the antenna support 17 and the antenna feed 27 is a radio frequency wobble joint 16 in the wave guide 13 which permits energy to be delivered from the translating device 11 to the antenna feed 27. The nutating mechanism 12 causes the energy from the translating device 11 to sweep across the reflector 10 at a predetermined rate dependent upon the period of nutation. The nutating mechanism shown is merely illustrative of the type of apparatus used, and any of several well known types which rotate or nutate the antenna feed will suflice.

The antenna feed 27 may be of any suitable type, however, the one shown is disclosed in the United States Patent 2,422,184 to Cutler and hence is not described in detail.

A metallic shield 23 has a continuously curving surface 24 so thatthe shield has a constantly varying slope. The shield 23 is positioned between the nutating feed 27 and the reflector 10 and is provided at one end with an aperture 25, which is just large enough to permit the antenna feed 27 to emerge. At the other end of the shield 23 is an aperture or mouth 26, which is of a diameter larger than that of aperture 25 so that the shield fits snugly over a housing 22 which protrudes through the reflector 10. The shield 23 will thus house the gimbal ring 19 and the wobble joint 16 and will protect the other parts of the nutating mechanism 12.

The continuously curving contour 24 of the meallic shield 23 diffuses the energy which is being delivered to or from the antenna feed 27 so that no energy is reflected back into the antenna feed 27 from the gimbal ring 19 or other parts of the nutating mechanism 12 to cause an impedance mismatch. The energy which impinges upon the shield 23 is reflected at an infinite number of different angles so that any of the energy, which, if further reflected from the reflector 10, will not disturb the relationship between the major and minor lobe characteristics of the parabolic reflector 10. Thus the shield 23 will not aifect the minor lobes inherent in the parabolic reflector 10.

The length of the shield 23 must be several wave lengths of the frequency being used so that the shield 23 is physically large enough to shield the nutating system 12. The use of a continuously curving shield having a length of several wave lengths permits the antenna system to function properly over a very wide frequency band at all times.

It is to be understood that the above described arrangements are merely illustrative of the application of the principles of the invention, and that numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. In a radar antenna system,'a parabolic reflector, a movable antenna feed, means to move the feed, and a shield having a constantly varying longitudinal peripheral slope and located between the reflector and the feed and opening toward the reflector to prevent an impedance mismatch between the feed and the reflector.

2. In an ultra-high frequency antenna system, a parabolic reflector, a movable antenna feed, means for moving said feed, and a shield several wave lengths long and having a constantly varying longitudinal slope on the exterior, said shield being in the radiation path between the feed and the reflector and opening in the opposite direction as the reflector.

3. An antenna system comprising a stationary parabolic reflector, a movable feed for transmitting and receiving ultra-high frequency energy, means for moving the feed, and a protective impedance mismatch preventing shield several wave lengths long, said shield partially covering the feed and the feed moving means and located in the path of radiation between the feed and the reflector to deflect any energy impinging on the shield away from the feed and to the reflector, said shield presenting, on a longitudinal cross section, two symmetrical convex sides having continually changing slopes extending between two straight ends, one of which is larger than the other, the larger end facing the parabolic reflector.

4. An ultra-high frequency antenna system comprising a feed horn; a parabolic reflector; means to move said feed horn relative to said reflector so that the antenna will scan; and a shield having a constantly changing longitudinal peripheral slope and an aperture at one end so that the said horn may extend therethrough, said shield having a second aperture, said second aperture facing the reflector and larger in diameter than said first aperture in order to position the shield around said feed moving means for protection thereof, said shield serving to prevent an impedance mismatch between the feed horn and the reflector.

5. In an ultra-high frequency antenna system, a parabolic reflector to direct ultra-high frequency energy, an antenna feed movable with respect to said reflector, means to move said feed, a shield surrounding said feed moving means and that part of the feed adjacent the concave side of the reflector, said shield having a constantly varying slope and opening in an opposite direction from the parabolic reflector.

6. In an ultra-high frequency antenna system, a movable antenna scanner for transmitting energy, means for moving the scanner, a housing for the moving means, a stationary parabolic reflector having a central opening through which the said housing and a portion of the moving means of the scanner protrude, a constantly curving shield several wave lengths long positioned between a portion of the movable scanner and the stationary reflector and opening toward the reflector, said shield having at one end an opening through which the scanner protrudes and at the other end an opening Which fits over the said housing so that the shield is positioned within the path of radiation to ditfuse any energy received from the scanner to prevent impedance mismatching between the scanner and the reflector to increase antenna gain and to avoid distortion of the radiation pattern of the reflector.

7. In an antenna system; a stationary reflector; a movable feed means for transmitting and receiving ultrahigh frequency energy; means for moving the feed means; and a metallic shield several times longer than the wave length of the energy transmitted and/or received and having two convex sides of constantly changing slope along its longitudinal cross section, said shield opening toward the reflector and having an aperture through which the feed means projetcs so that the shield covers and protects portions of the feed means and the feed moving means, said shield being located in the path of radiation and serving to prevent impedance mismatching between the reflector and the feed means by reflecting incident energy from the feed to the reflector.

8. In an ultra-high frequency antenna system, a parabolic reflector having a central aperture, a movable antenna feed projecting through the aperture in the reflector, means to move the feed, a housing for the feed moving means, said housing projecting through the aperture in the reflector and surrounding a portion of the feed, a shield opening toward the reflector and having one end fitted over the housing, said shield having an aperture at the other end through which the feed means projects.

References Cited in the file of this patent UNITED STATES PATENTS 2,446,436 Rouault Aug. 3, 1948 2,492,951 Baker et al. Jan. 3, 1950 2,535,699 Schnell et al. Dec. 26, 1950 

